Optical disk devices utilize laser light for recording data onto and sensing data from storage media. These devices are often used for the storage of computer-prepared data and have recognized value in their ability to store large quantities of data. The media for use in such devices is reactive to light and is heated thereby to levels which enable the recording of data. To write data on optical media, a laser beam is focussed onto the media surface and the laser is operated at a relatively high power level in order to alter the media in accordance with an input data stream. In reading data back, the laser power level is controlled to a lower level so that the media is not altered by the laser beam, but the reflected light indicates the presence or absence of media alterations.
Optical media is of three general types, media which can be written only once, media which can be written, erased and written again and media which is read only such as CD-ROM. This invention relates to systems which employ erasable media such as magneto-optic (MO) media. Such media is not permanently altered when the data is written. In MO media, the magnetic orientation of the reactive material is altered in the writing process, and in the erasing process, the magnetic orientation is reordered.
In operating an optical disk system, it is necessary to set the correct laser power level to read and to write for each optical disk. The correct parameters for the optical disk are included in information in an identification header written onto the disk itself. That information, when read by the system, enables a calibration circuit to set the desired current levels for the laser to produce correct laser power. Since, however, the laser is subject to unintended changes in its operating parameters, particularly with temperature and aging, a calibration system is also used to change current levels for the laser so that the correct power level is maintained under operating conditions and throughout laser life.
The common practice of calibrating laser circuits to operate with a given optical medium usually involves analyzing the laser light intensity at the optical medium at the time the optical disk device is manufactured. To perform the analysis, laser control circuits are set to match a predetermined desired light intensity at the optical medium. Analysis is conducted to establish data for entry into the device memory. That data is used by the device processor to enable setting digital to analog converters (DACs) which control laser power in the writing and erasing operations. Read power levels and the power level to write "zero" digits, called a baseline level, are also established under control of the device processor. In a typical optical disk device, the baseline level for data recording may be 0.5 milliwatts, while the instantaneous write power level may be established at 5 to 20 milliwatts during the time space for a single write event, that is, the time needed to write a single mark on the media. The laser may be rapidly switched from the higher level to the baseline level several times during a single write event. It is not unusual to write with several short high-power pulses interspersed with longer periods at the baseline level.
In a typical optical device, the read power level may be set at 2 milliwatts, while the erase power level, during which the light source operates continuously, may be set at 8 milliwatts.
Semiconductor diode lasers are presently the light source of preference in optical disk systems. They are light weight, efficient in electrical-to-optical power conversion, and can be intensity modulated by control of the injection current. Because of losses in beam collimating, circularizing and focusing elements, the optical path efficiency for coupling the generated light to the disk, is about 50%. Thus, the power requirements of the light source are twice those required at the disk and 50 milliwatt diode lasers are typically employed.
The stability of the laser emission is a major factor in determining the signal-to-noise ratio for data retrieval. Diode lasers are subject to mode hops between Fabry-Perot cavity resonances and are therefore subject to an inherent instability. In addition, light reflected from the optical disk directly back to the laser greatly increases laser instability. In magneto-optic (MO) systems, such feedback cannot be eliminated with passive optical components because the signal is detected as elliptical polarization produced at the disk surface by Kerr rotation of incident linearly polarized light. However, increasing the power level at which the laser operates to 15-20 milliwatt pulses, significantly reduces its sensitivity to feedback. Nevertheless, the pulse-to-pulse repeatability of write power levels is of concern, and is one of the major causes of failure to record data properly during a write event.